After the end of The Cold War, a peace dividend took place. After German unification, NATO promised no further Eastern expansion, but after the Russian invasions of Moldova and Georgia to liberate Transnistria and Abkhazia, which happened to have a large Russian population, other former Warsaw Pact nations wanted to join NATO.
Since the second invasion of Ukraine by Russia, Russia has been testing European defences. While Russia has claimed it is innocent, this is the same Russia that promised it would not invade Ukraine before invading it again.